Temperature swing adsorption molecular sieve units are used in a variety of industries to remove contaminants from liquids and gas streams. This is a batch-wise process consisting of two basic steps which are adsorption and regeneration. In the adsorption step, contaminants are removed by being adsorbed on the solid molecular sieve material and then the treated stream leaves the unit with contaminant levels below the required specification limit or further treatment is necessary. In the regeneration step, contaminants are desorbed from the solid molecular sieve material by means of a regeneration stream (typically gas).
The regeneration step consists of two major parts—heating and cooling. In the heating part of the process, the regeneration stream, which is contaminant free, is heated to an elevated temperature (290° C. in one embodiment of the invention) and flows over the molecular sieve material. Due to the heat of the gas, mainly used as heat of desorption, and the difference in partial pressure of the contaminants on the molecular sieve material and in the regeneration gas stream, the contaminants desorb from the solid material and leave the unit with the regeneration gas. A cooling step is then necessary. As a result of the heating step the molecular sieve material heats up. To prepare the material again for the next adsorption step and since adsorption is favored at lower temperatures than desorption, the molecular sieve material needs to be cooled by means of a stream typically flowing over the molecular sieve at a temperature very close to the feed stream temperature.
Hence, the most basic form of temperature swing molecular sieve process unit consists of two vessels with one vessel in adsorption mode and the other vessel in regeneration mode. However, dependant on the amount of the feed stream to be treated as well the amount of contaminants to be removed from the feed stream, several vessels, which operate in a parallel mode, could be required. In a more complicated form of operation, the regeneration step can also be split over two vessels in a series-heat-and-cool cycle, where one of the vessels would be in the heating step and another would be in the cooling step.
Apart from the basic adsorption and regeneration steps described above, additional steps may need to be included dependent on the pressure levels of the feed stream versus the regenerant stream. For instance, if adsorption is carried out at a higher pressure than regeneration (note that a lower pressure will favour desorption of contaminants from the molecular sieve material), at a minimum, two additional steps are required: a depressurization step where the pressure is reduced from adsorption pressure to the regeneration pressure; and a repressurization where the pressure is increased from the regeneration pressure to adsorption pressure. Note that sometimes the opposite is true, with regeneration carried out at a higher pressure than adsorption, but in this case again a depressurization and repressurization step need to be included. If depressurization and repressurization steps are present they are typically part of the regeneration cycle.
All of the above steps are typically programmed into a so-called “switching sequence”, either in a Programmable Logic Controller (PLC) or Distributed Control System (DCS) to allow this in essence batch-process to work as a semi-continuous process.